This invention relates to capacitive pressure transducers having a stationary electrode and a movable diaphragm and, more particularly, to a capacitive pressure transducer which can compensate for thermally induced changes in sensitivity and initial capacitance.
Capacitive pressure sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor and a deformable conductive member, such as a metal foil diaphragm, which forms the other plate of the capacitor. Generally, the diaphragm is edge-supported, having a central portion that is movable and positioned substantially parallel to and opposite the fixed plate. Since the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance of the sensor is inversely proportional to the gap, d, between the central portion of the diaphragm and the conductive surface of the fixed element. In order to provide a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed or pneumatically separated from the region on the opposite side.
In practice, the geometry and physical characteristics of the diaphragm are selected so that the pressure differentials across the diaphragm in a particular range of interest cause predetermined displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the transducer. For relatively high sensitivity, such transducers require large changes of capacitance in response to relatively small gap changes. To achieve such sensitivity from unit to unit, nominal gap dimensions need to be very small and generally require that their component parts be manufactured to very close tolerances to establish the required dimensional relationships. In addition, the structure and materials must maintain those relationships over a useful temperature range.
One type of pressure transducer ("tensioned diaphragm transducer") includes a tensioned diaphragm that is peripherally supported by the rim of a concave body member of the transducer, where the diaphragm is relatively thin and it is maintained under radially directed tension which is directly related to the pressure deflection sensitivity of the diaphragm. Another type ("bending diaphragm transducer") has a generally similar structure, but the diaphragm is relatively thick and is not maintained in tension. The pressure deflection relationship depends on the bending rigidity of the diaphragm. Both types are typically configured to provide changes in capacitance with changing pressure across the diaphragm. In both types of transducers, the diaphragm and the body member are ideally constructed of materials having identical coefficients of thermal expansion. However, in practice that does not occur. As a result, changes in temperature cause the diaphragm and the body member to expand or contract at different rates thus causing effects on transducer output. For tension diaphragm transducers, there are two effects which affect the accuracy of the transducer. The first effect is the change of sensitivity of a sensor due to change of temperature. The sensitivity is the slope of the calibration curve of a transducer. For a capacitive type pressure transducer with a tensioned diaphragm, the sensitivity is related to the motion of the diaphragm per unit pressure. The second effect is the change of "zero" of a sensor due to the change of temperature. The "zero" is the output of a transducer at zero pressure or some other reference pressure. For a capacitive transducer, the "zero" is related to the initial value of the capacitance when the pressure is zero, or some other designated pressure. Initial capacitance is related to the initial gap, area of the electrode as well as other factors such as the leakage capacitance across the insulation material. For bending diaphragm transducers, only the "change of zero" effect is present. The sensitivity change with temperature is generally caused by the change of modulus of elasticity of the diaphragm material and generally not very significant.
FIG. 1 shows an exemplary calibration curve for a transducer at two different temperatures T1 and T2. As shown for each temperature, the output is substantially linear with pressure, with sensitivity equal to S1 at T1 and S2 at T2, yielding a temperature induced "change in sensitivity" equal to S2-S1 for the transducer. For the example of FIG. 1, at pressure equal to zero, there is a difference "Z" in output, representing the temperature induced "change in zero" for the transducer. It is desirable that there is no change in sensitivity and zero over the operational range in temperature for a transducer.
Accordingly, it is an object of the present invention to provide an improved pressure transducer.
Yet another object of the present invention is to provide an improved pressure transducer that is relatively inexpensive and easy to manufacture.
Still another object of the present invention is to provide an improved pressure transducer which compensates for thermally induced changes in sensitivity.
A further object of the present invention is to provide an improved pressure transducer which compensates for thermally caused changes in the output of a transducer at a predetermined (or zero) ambient pressure.